(1) Field of the Invention
The present invention relates to an intake air flow rate detector of an internal engine and more particularly relates to an intake air mass flow rate detector using Karman vortex sensor.
(2) Description of the Prior Art
A Karman vortex sensor comprises a column shaped vortex generator arranged in a flow of a medium and is based on a principle that the frequency of the generated Karman vortex is in proportion to the flow velocity of the flowingmedium. The Karman vortex sensor is suitable for use as a detector of the amount of intake air due to its simplicity in construction and its small resistance for the flow of the intake air.
One drawback of the Karman vortex sensor is that it requires a compensation by mass of the flowing medium or the air. This is by a reason that the detected signal of a Karman vortex sensor is in proportion to the volumetric flow rate Qv, whereas the amount of the intake air of an internal engine should be detected by the mass flow rate Qw so a compensation or calibration is required.
As a conventional method for detecting the mass flow rate Qw it has been known to use a Karman vortex sensor in combination with a dynamic pressure detector for instance a Pitot tube. The volumetric flow rate Qv is detected by the Karman vortex sensor and the dynamic pressure .rho.Q.sup.2 v (where .rho. is density of the air) is detected by the dynamic pressure detector. By using an operational circuit and by making dividing operation the mass flow rate Qw is obtained according to a formula: ##EQU1##
In an internal combustion egine for an automobile, the amount of the intake air varies greatly and the range of the variation is very wide so that there will be a comparative portion in said wide range in which an error by the above measurement becomes substantially large.
Namely the range of generation of the Karman vortex of the Karman vortex sensor depends on the velocity of the flow (Reynolds number). The range in which a stable Karman vortex train appears is about 3,000 to 45,000 in Reynolds number. Beyond this range, the instability of the Karman vortex tends to increase. Whereas the range of measurement of the intake air of an internal engine of an automotive vehicle is quite large as 1,200 to 65,000 by Reynolds number in case of an engine having a displacement of 2,000 cc and the width of the vortex number is 20 mm. Therefore at portions located at both extremities of the range of the measurement, the value obtained may become instable. Furthermore, since the dynamic pressure is in proportion to the square of the volumetric flow rate, the level becomes very small at the flow rate range and a large measurement error results. If the mass flow is calculated by dividing operation of the two values obtained at such circumstances, the error at high flow rate and low flow rate (especially the more for the latter) becomes large mainly due to error in the measurement of the dynamic pressure and unstable rotation of the engine may be caused.